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I am not sure if this is what you want, but if you add some number of zeros in the end of your input list you will get higher resolution (just more points) in the output. This trick works best if your signal decays to zero in the end (and is zero at the beginning). It does not, however, improve the actual resolution of the FFT, for example in case the characteristic frequency in your signal is not an exact multiple of the sampling rate. In the latter case, you can use Hamming window for example to improve resolution.